The present invention relates to a filtering device using the transfer of electrical charges in semiconductor and having two electrical charge transfer filters, one being recursive and the other non-recursive.
An electrical charge transfer filter generally comprises:
a shift register for the transfer of electrical charges made on a semiconductor substrate covered by an insulating layer on which transfer electrodes are arranged parallel to one another and transversely with respect to the direction of the charges. For a periodic application of given potentials, these electrodes ensure the transfer into the substrate, from one register stage to the next, of groups of charges corresponding to samples of the input signal to be filtered;
weighting means connected to the output of the shift register stages which assign a weighting factor to the quantity of charges having passed through each stage.
The quantities of weighted charges are generally read differentially and this differential signal constitutes the electrical output signal of the filtering means.
In a recursive filter this signal is injected again at the input of the filter after having generally been phase-displaced by 180.degree..
In a non-recursive filter, there is no reinjection of the output signal at the filter input.
To bring about a given filtering in the case of a non-recursive filter, it is necessary to have a large number of weighting factors and therefore shift register stages and in the case of a recursive filter maximum accuracy in the values of the weighting factors which leads to construction problems.
Hybrid filtering devices for the transfer of electrical charges, i.e. having two electrical charge transfer filters, one recursive and the other non-recursive, have the advantage of reducing the number of weighting factors and the accuracy again required for the factor values for a given filtering operation.
Such filtering devices are known in the prior art where the two charge transfer registers belonging to two filters are arranged in series on the same semiconductor substrate. The connection between the two filters is made by charge transfer. Means belonging to each filter ensure the reading of quantities of weighted charges.
The first filter is recursive and the signal from its reading means is transmitted, after phase displacement, to means for the injection of charges located at the filter input, which also receives samples of the input signal.
The second filter is non-recursive and the signal from its reading means constitutes the output signal of the filtering means.
The disadvantage of such known hybrid devices is that it is necessary to have weighted charge quantity reading means for each filter. Devices for reading charges, in current or in voltage are known, in the prior art. The disadvantages of these devices in their complexity and overall dimensions, which are greater than those of the charge injection means.